A CyberKing was a Dreadnought-class ship whose pilot was integrated into it, making it the CyberKing. It was a huge bipedal robot operated by a living controller. These ships were used to front a Cyberman invasion by crippling whole cities and upgrading their populations en masse. They were the largest, if not the most powerful, weapons utilised by the Cybermen in battle.
One CyberKing was created by the Cybermen of Cybus Industries in London in 1851. The Tenth Doctor theorised that the end of the 2009 Dalek invasion of Earth collapsed the walls of the Void, allowing the Cybermen to escape and travel back in time using a stolen Dalek Dimension Vault. Using child labour to generate electricity and produce the ship underneath the River Thames, Mercy Hartigan was made into the CyberKing. She refused to be cyber-converted, and instead used her mental powers to command the entire Cyberman army. The ship rose out of the Thames, with Miss Hartigan and several Cybermen controlling the machine inside the part of the vehicle shaped as a Cyberman mouthpiece, and rampaged across London. Using Jackson Lake's "TARDIS" and an improvised device made from Cyber infostamps, the Doctor severed the link between Hartigan and the Cybermen. The Cybermen and Miss Hartigan were killed in the process, and the CyberKing was transported into the Time Vortex to harmlessly disintegrate. (TV: The Next Doctor)
It was later revealed that this event was not remembered by the people that witnessed it due to the cracks in time, which had erased events such as this from history. (TV: Flesh and Stone) When the Twelfth Doctor corrected a Coal Hill School history book, referencing the CyberKing's attack on London was one of the many additions he made, sketching its likeness in the section about the Victorian era. (PROSE: A History of Humankind)
In an alternate timeline where the Cyberiad invaded all of time and space, at least three CyberKings led a full-scale invasion of Sontar, their firepower overwhelming even the Sontarans' defences. The Tenth Doctor became integrated within a CyberKing in an attempt to repel the invasion, but he was nonetheless defeated by a Cyber-Ark. (COMIC: Supremacy of the Cybermen)
The CyberKing control unit was converted using a side handle headset that connected the victim's mind to the CyberKing, changing their speech patterns and their eye colour throughout the process. Once the conversion was complete the controlling unit could operate the CyberKing's movements and in the case of Miss Hartigan could even take command of the Cybermen themselves. The CyberKing ship was a massive Cyberman-shaped walker, using its two legs to stride through the battlefield. It had a weapon attached to each arm; a laser blaster on its left arm, and a mortar launcher on its right arm. The head served as the command centre for the vehicle, while its body contained a large cyber-conversion unit, allowing it to transform organic life into new Cybermen. The Cyberking had some components similar to Victorian machinery, to the extent that it appeared to be powered by steam engine, with smokestacks protruding from the shoulders. This suggests that the CyberKing was repaired by the Cybermen after exiting the void using the technology of the time (like the cybershades and the "engine" that started the CyberKing). If this is the case, it can not be determined how much of the CyberKing's features are shared with an original Dreadnought-class ship. (TV: The Next Doctor)
The CyberKings of the Cyberiad possessed a similar, but far more advanced, design to their Cybus counterparts. Unlike their Cybus counterparts however, these CyberKings had no gaps in their armor with the running lights glowing the same piercing shade of blue as the Cyberiad's basic units. (COMIC: Supremacy of the Cybermen)
Behind the scenes
A noticeably smaller CyberKing (acting as an individual rather than a ship) was featured as a minor antagonist in the video game LEGO Dimensions. After most of its body was destroyed, this CyberKing displayed the ability to scuttle around as a head, not unlike what a regular Cyberman had done in The Pandorica Opens.